A Global Navigation Satellite System (GNSS) transmits ranging signals, which are used by land, sea and air users to determine a three-dimensional position, a velocity and a time of day. The GNSS includes a network of satellites to transmit the ranging signals. One example of a GNSS is a Global Positioning System (GPS). The GPS includes 24 satellites, called GPS satellites, which operate in medium earth orbit.
The GNSS may be accurate to no more than 10 meters due to errors such as from ephemeris data, satellite clocks, ionosphere, troposphere, multi-path and receiver noise. Thus, the GNSS by itself may not be accurate enough for certain users that require better accuracy, for example, aircraft. Other systems called augmentation systems adjust for these errors and broadcast them to users. A typical augmentation system for a GPS includes one or more reference stations having GPS receivers that receive GPS messages (i.e., ranging signals) from the GPS satellites. The precise locations of the reference stations are known. A master control station receives the GPS data from the reference stations and determines for each GPS satellite the bias based on the reference station's known location and the GPS satellite's determination of the reference station's position. The master control station determines a range error for each GPS satellite, and broadcasts the integrity data and corrections, which will eventually be received by a user.
One known augmentation system is the satellite-based augmentation system (SBAS). In the SBAS system, the master control station sends integrity data and corrections to geosynchronous earth orbit (GEO) satellites for broadcast to the users on the same frequency and in a similar format to GPS satellites. This method allows both SBAS GEO signals and GPS signals to be received via the user's GPS receiver, and has the added benefit of providing additional satellite ranging sources to improve availability. Wide-area coverage is only limited by the footprint of the SBAS GEO satellite (which spans approximately +/−76 degrees in longitude and latitude) and the quantity and location of the SBAS reference stations within the GEO footprint (i.e., range). Thus, a single SBAS may be used to service a nation or a continent.
Another known augmentation system is the ground-based augmentation system (GBAS). A typical GBAS includes a series of reference stations having GPS receivers located around a local area (e.g., 20 or 30 miles). The GPS receivers forward the GPS data to the master control station, which processes the GPS data and broadcasts the information using a very high frequency (VHF) data broadcast to an aircraft. The GBAS can provide local area augmentation with integrity data and corrections for GPS satellites to users in the immediate vicinity of an airport via a VHF data link. Since the GBAS is utilized in a local area, a much higher degree of fidelity may be placed on the accuracy and integrity information provided to the aircraft for navigation, and a much more rapid response to fault conditions (i.e., time-to-alert) can be achieved.
A further known augmentation system that does not use a reference station is the air-based augmentation system (ABAS). The ABAS typically uses the GPS signals from GPS satellites and user avionics to augment the GPS signal. The ABAS uses a Receiver Autonomous Integrity Monitoring (RAIM) to perform integrity monitoring. Since the RAIM is based on comparing the results of multiple combinations of GPS satellites, the ABAS requires more GPS satellites in view than the minimum four required for basic position fixing. Hence, the ABAS gains its additional integrity at the expense of reduced availability. The ABAS may also utilize an airborne inertial navigation system (INS) to improve availability and continuity.